1. Field of the Invention
The present invention relates to a servo motor, especially to a servo motor with large rotor inertia.
2. Description of Prior Art
The major function of servo motor is to drive load and achieve a precise position for the load. The motor and load is generally connected by flexible link to couple the axes of motor and load. The flexible link uses the flexibility pro se to balance the strain and vibration cause by misalignment between motor and load. However, the motor and the load become a dynamic system of two degree of freedom. Therefore, the rotor inertia and the load inertia need good match to achieve good system controllability. The rotor inertia should be carefully chosen for the known load inertia to achieve optimal performance.
Moreover, the rotor inertia is proportional to rotor length and fourth power of rotor radius. There are two approaches for servo motor with the same cross section. The first approach increases rotor length as shown in FIGS. 1A and 1B. The motor comprises a casing 10a, a stator 11a fixed in the casing 10a, and a rotor 12a with both ends connected to front and rear sides of the casing 10a through bearings. The rotor 12a comprises magnetic body at center thereof and corresponding to inner end of silicon steel plates of the stator 11a. As shown in FIG. 1B, the rotor inertia is increased by increasing rotor length. However, the weight of the rotor 12a is increased too. The lifetime of bearing is reduced and the cost of magnetic body and silicon steel plate are increased because the length of the stator 11a is also increased.
Another approach is to increase the outer diameter of rotor. As shown in FIGS. 2A and 2B, the motor comprises a casing 20a, a stator 21a arranged in the casing 20a and a rotor 22a fixed to front and rear sides of the caseing 20a through bearing. The rotor 22a comprises magnetic body on center thereof and corresponding to inner side of the silicon steel of the stator 21a, where the outer diameter of the rotor 22a is increased to increase the inertia of rotor, as shown in FIG. 2B. Therefore the size of the silicon steel of the stator 21a needs increment and the mold for the silicon steel of the stator 21a also needs redesign. For a production line providing various rotor inertias, stator 21a with silicon steel of various sizes are required. The number of mold die for the stator 21a is also increased and the cost is also increased.